1. Field of the Invention
The present invention relates to an ink jet printhead, and, more particularly, to a method and apparatus for predicting and limiting maximum printhead chip temperature in an ink jet printer.
2. Description of the Related Art
In an ink jet printer, excessive print density can cause several problems within the printer. One of these problems is excessive print chip temperature. In order to avoid excessive chip temperature, the printer must pre-analyze data and be able to predict the temperature of the print chip.
In previous ink jet printers, the basic approach was to count the number of drops in the next pass of the printhead. If the drop count were too high, the single pass would be broken into multiple passes. This previous approach does not accurately predict the print chip""s temperature. When the print system is composed of an ink jet chip that is mounted on a metal heatsink and the chip has built-in substrate heaters which are used to regulate temperature, a new approach to prediction is required.
In an ink jet printer, the drop and mass of the ink are dependent upon the temperature of the head. If the temperature of the head varies significantly from swath to swath, then a color shift will become visible, a phenomenon which is referred to as xe2x80x9cbanding.xe2x80x9d In order to overcome this problem, ink jet printers typically add heat to the print chips by the use of substrate heaters. By attaching the print chip to a metal heatsink, swings in chip temperature can be further reduced.
As ink jet printers begin to move into the business market, it becomes necessary to manage the printhead in a more efficient manner. Even with the addition of heat and the attachment to a metal heatsink, due to the number of nozzles present on the chip, it is still possible to cause excessive swings in chip temperature when printing high-density images. This requires that the printer be able to predict the temperature of the print chips for future swaths.
The present invention provides a method of accurately determining whether a temperature of a print chip in an ink jet printer will exceed a predetermined limit temperature based upon a number of ink drops to be emitted, and, if so, reducing the number of ink drops to be emitted such that the predetermined limit temperature is not exceeded.
The invention comprises, in one form thereof, a method of controlling a temperature of a print chip of a printhead in an ink jet printer. A memory device is provided within the printer. Ink is emitted from the printhead. Temperature data associated with the print chip during the emitting step is recorded. A thermal resistance value associated with the printhead and/or a thermal capacitance value associated with the printhead is calculated. The calculating is dependent upon the recorded temperature data. The thermal resistance value associated with the printhead and/or the thermal capacitance value associated with the printhead is stored in the memory device. The memory device also sets the amount of energy applied for each drop. A temperature of the print chip at a future point in time is estimated based upon a number of ink drops to be emitted by the printhead before the future point in time, and the thermal resistance value associated with the printhead and/or the thermal capacitance value associated with the printhead. The estimated temperature is compared to a predetermined limit temperature. If the estimated temperature exceeds the predetermined limit temperature, the number of ink drops to be emitted by the printhead before the future point in time is reduced.
The invention comprises, in another form thereof, an ink jet printer including a printhead having a print chip. A memory device stores a thermal resistance value associated with the printhead and/or a thermal capacitance value associated with the printhead. The memory device also sets the amount of energy applied for each drop. A controller retrieves the thermal resistance value associated with the printhead and/or the thermal capacitance value associated with the printhead from the memory device. The controller calculates a maximum temperature of the print chip during printing based upon the thermal resistance value associated with the printhead and/or the thermal capacitance value associated with the printhead.
An advantage of the present invention is that the temperature of the print chip can be more accurately predicted.
Another advantage is that the temperature of the print chip can be more reliably prevented from exceeding a predetermined limit temperature.
The method of the present invention incorporates the effect of the metal heatsink into the prediction of chip temperature. Data analysis occurs in groups that are much smaller than the thermal time constant of the system. While most ink jet printers simply count the number of drops in a swath and decide what action to take based on that count, the method of the present invention includes analyzing drop counts in groups that are smaller than an entire swath. A swath is commonly known as the set of print data that can possibly be printed in one complete pass of a printhead across a print medium. In order to improve print quality, however, the actual printing of a swath of data is often spread out over the course of multiple passes of the printhead across the print medium. Analyzing drop counts in groups that are smaller than an entire swath improves the accuracy of maximum temperature prediction.
Information contained in the memory device associated with the printhead is used to establish an approximate amount of energy per drop of ink. By multiplying the drop count per group by the energy per drop, the total energy per group is computed. The power required per group is computed based on the time required to print each group.
The print chip""s response is predicted with a simple exponential model that contains thermal parameters that are not constant. The thermal parameters determine the chip""s response to heating and describe how the chip cools. These parameters are based on two items: the power applied for each group and the difference between the chip""s target control temperature and its heatsink temperature.
A calibration sequence occurs in the machine whenever a new printhead is installed. The printhead is jetted at a known fire rate and temperature data is recorded. This information is used to adjust the nominal thermal parameter tables stored in the machine.
By applying a difference equation, the change in chip temperature across the swath is predicted. Based on the maximum predicted temperature, the printer can shingle the entire page at a higher rate or switch to a higher shingle rate within the page.